Light scanner, multibeam scanner, and image forming apparatus using the same

ABSTRACT

A multibeam scanner including a light source for emitting a plurality of light beams, a deflector for deflecting the plurality of light beams emitted from the light source, and a scanning optical system for guiding the plurality of light beams deflected by the deflector to a scan surface. A first light shielding member for determining one end of the diameter of at least one of the plurality of light beams and a second light-shielding member for determining the other end of the diameter of the at least one of the plurality of light beams are disposed apart from each other in a direction in which the light beams propagate and between the light source and the deflector. The first light-shielding member and the second light-shielding member are used to limit the diameter of the at least one of the plurality of light beams.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light scanner, a multibeam scanner,and an image forming apparatus using the same. More particularly, thepresent invention relates to an image forming apparatus which issuitable for use as a laser beam printer or a digital copying machine,making use of an electrophotographic process or the like, and which isused to record image information by deflecting (reflecting) light whichhas been emitted from a light source using a polygon mirror, serving asdeflecting means, and by scanning a scan surface with the light throughscanning optical means (that is, an image-forming scanning opticalsystem). Even more particularly, the present invention relates to amultibeam scanner which achieves higher speed and higher definition byperforming optical scanning operations using a plurality of light beamsat the same time, and which provides a good image by reducing jittersand pitch errors.

2. Description of the Related Art

FIG. 18 is a sectional view (main scanning sectional view) of the mainportion of a related multibeam scanning optical system in a mainscanning direction thereof.

In FIG. 18, reference numeral 91 denotes light source means, which is,for example, a semiconductor laser array including two light-emittingpoints (light sources). The two light-emitting points are disposed apartfrom each other in the main scanning direction and a subscanningdirection. Reference numeral 92 denotes a condenser lens system whichincludes one collimator lens and which converts two light beams thathave been emitted by the light source means 91 into substantiallyparallel light beams or convergent light beams. Reference numeral 93denotes a cylindrical lens which has a predetermined refractive poweronly in the subscanning direction. Reference numeral 94 denotes anaperture diaphragm which shapes the two light beams that have passedthrough the cylindrical lens 93 so that they have desired optimalshapes. Reference numeral 95 denotes deflecting means (light deflector),which is, for example, a rotating polygon mirror, and which rotates at aconstant speed in the direction of arrow A by driving means 98 such as amotor. Reference numeral 96 denotes a scanning lens system(image-forming scanning optical system), serving as scanning opticalmeans, having an fθ characteristic. The scanning lens system 96 includestwo fθ lenses, a first fθ lens 96 a and a second fθ lens 96 b. Thescanning lens system 96 has a tilt correcting function as a result ofputting a location near a deflecting surface 95 a of the light deflector95 and a location near a photosensitive drum surface 97, serving as ascan surface, in a conjugate relationship within a subscanningcross-sectional plane.

In FIG. 18, the two light beams that have been emitted by the lightsource means 91 after being modulated in accordance with imageinformation are converted into substantially parallel light beams orconvergent light beams by the condenser lens system 92, and theconverted light beams are incident upon the cylindrical lens 93. Of thelight beams portions incident upon the cylindrical lens 93, those withina main scanning cross-sectional plane exit from the cylindrical lens 93unchanged, while those within the subscanning cross-sectional plane arefocused in order to form a substantially linear image (a longitudinallinear image in the main scanning direction) on the deflecting surface95 a of the light deflector 95 through the aperture diaphragm 94. Here,by the aperture diaphragm 94, the cross-sectional sizes of the lightbeams are limited. The trio light beams that have been deflected(reflected) at the deflecting surface 95 a of the light deflector 95 arefocused in the form of a spot on the photosensitive drum surface 97 batthe scanning lens system 96. By rotating the light deflector 95 in thedirection of arrow A, the photosensitive drum surface 97 is opticallyscanned at a constant velocity in the direction of arrow B (mainscanning direction). By this, two scanning lines are formed on thephotosensitive drum surface 97, which is a recording medium, in order torecord an image.

In order to record image information with high precision using this typeof multibeam scanner, it is important to properly correct jitters(displacements in printing positions) and non-uniform pitches over theentire scan surface by properly focusing a plurality of light beams onthe entire scan surface.

In general, when forming an image by scanning the photosensitive drumsurface with light beams that have been emitted by the light source, inorder to obtain a good image with high resolution, it is necessary toreduce the diameter of a light beam spot on the photosensitive drumsurface and to reduce the pitch in the subscanning direction.

In order to reduce the pitch in the subscanning direction in themultibeam scanner, light source means (or a semiconductor laser array)disposed by being tilted obliquely from the main scanning direction isoften used.

FIG. 19 is a sectional view (main scanning sectional view) of anotherrelated multibeam scanner of this type in a main scanning directionthereof. In FIG. 19, component parts corresponding to those shown inFIG. 18 are given the same reference numerals.

In FIG. 19, since a plurality of light-emitting points 91 a and 91 b oflight source means 91 are disposed apart from each other by a certaindistance in the main scanning direction, light beams that have exitedfrom a condenser lens system 92 are not parallel to each other, so thatthey are at a certain angle from each other. Each light beam that hasexited from the condenser lens system 92 is incident upon a polygonmirror 95, which is a light deflector, through a cylindrical lens 93.

At this time, the light beams cross each other at the location of anaperture diaphragm 94 disposed between the condenser lens system 92 andthe polygon mirror 95, so that, by the angle of each light beam and by adistance L from a reference position of a deflecting surface 95 a of thepolygon mirror 95 to the aperture diaphragm 94, the interval between thelight beams on the deflecting surface 95 a of the polygon mirror 95 isdetermined (restricted). By reducing the interval between the lightbeams on the deflecting surface 95 a of the polygon mirror 95, the lightbeams are properly focused on a photosensitive drum surface 97.

A multibeam scanner which satisfies such optical characteristics isdisclosed in, for example, Japanese Patent Laid-Open No. 5-34613.According to this document, in the structure of the multibeam scanner, aplurality of light beams are converted into substantially parallel lightbeams at a condenser lens system, and the substantially parallel lightbeams are caused to impinge upon a polygon mirror through an aperturediaphragm. Then, by a scanning lens system, the substantially parallellight beams are led onto a scan surface. When scanning the scan surfacewith the plurality of light beams at the same time, the relationshipamong the number of light-emitting points of the light source means, thepitch in a main scanning direction, the distance from the polygon mirrorto the aperture diaphragm, and the focal length of the condenser lenssystem is specified in order to properly focus the plurality of lightbeams on the scan surface.

When a multibeam scanner is used, it is necessary to properly correctjitters and pitch errors. Jitter refers to a relative displacement inpositions for printing using a plurality of light beams in a mainscanning direction. Pitch error refers to a deviation from a specifiedvalue (for example, 42.3 μm when printing resolution is 600 dpi) of theinterval between scanning lines that are formed when a plurality oflight beams are used at the same time for light scanning.

In order to reduce jitters, it is necessary to cause the light beams toreach the same location of the scanning lens system, or locations closeto each other on the scanning lens system, when scanning the samelocation of the scan surface. This is achieved by reducing the intervalbetween the light beams on the deflecting surface of the polygon mirror.Here, for example, a method for disposing an aperture diaphragm, servingas optimal means, near the deflecting surface of the polygon mirror isused.

However, more compact and wider-field-of-angle scanners in recent yearshave caused the scanning lens system and light beams deflected by thepolygon mirror to be disposed near the deflecting surface of the polygonmirror, so that there is no space to dispose the aperture diaphragm.Therefore, there is a problem in that it is physically difficult todispose the aperture diaphragm near the deflecting surface of thepolygon mirror.

In general, pitch errors are corrected by causing the magnification ofthe scanning lens system in the subscanning direction to be constant.However, pitch errors sometimes occur by, for example, decentering ofthe cylindrical lens and the scanning lens system in the subscanningdirection.

Here, in order to reduce how readily pitch errors are affected bydecentering, it is necessary to cause the light beams to reach the samelocation of the scanning lens system or locations close to each other onthe scanning lens system when scanning the same location of the scansurface. This can be achieved by reducing the interval between the lightbeams on the deflecting surface of the polygon mirror. Here, the methodfor disposing an aperture diaphragm, serving as optimal means, near thedeflecting surface of the polygon mirror is used. (This method is thesame as that used to reduce jitters.) However, the same problem as thatmentioned above arises.

On the other hand, when jitters and pitch errors are reduced bydisposing the aperture diaphragm near the deflecting surface of thepolygon mirror, the polygon mirror becomes larger, and the scanning lenssystem is disposed away from the polygon mirror. Therefore, increasedsize of the entire scanner becomes a problem.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a light scanner anda multibeam scanner which can properly focus light on a scan surface,and which can decrease jitters and pitch errors. It is another object ofthe present invention to provide an image forming apparatus using thesame.

It is still another object of the present invention to provide a lightscanner and a multibeam scanner which are compact, which have simplestructures, and which allow an aperture diaphragm and asynchronism-detecting diaphragm to be disposed at a greater number oflocations. It is still another object of the present invention toprovide an image forming apparatus using the same.

According to a first aspect of the present invention, there is provideda light scanner comprising a deflector for deflecting light emitted froma light source, a scanning optical system for guiding the lightdeflected by the deflector to a scan surface, a first light-shieldingmember for determining one end of a diameter of the light emitted fromthe light source, and a second light-shielding member for determiningthe other end of the diameter of the light emitted from the lightsource. The first light-shielding member and the second light-shieldingmember are used to limit the diameter of the light emitted from thelight source and are disposed apart from each other in a direction inwhich the light propagates.

In one form of the first aspect, the first light-shielding member andthe second light-shielding member are integrally formed.

In another form of the first aspect, the first light-shielding memberand the second light-shielding member limit a diameter of the lightwithin a main scanning cross sectional plane.

In still another form of the first aspect, the first light-shieldingmember and the second light-shielding member limit a diameter of thelight within a subscanning cross sectional plane.

According to a second aspect of the present invention, there is providedan image forming apparatus comprising a light scanner according to anyone of aforementioned claims, a photosensitive member disposed at thescan surface, a developing device for developing as a toner image anelectrostatic latent image formed on said photosensitive member usingthe light with which said photosensitive member has been scanned by thelight scanner, a transferring device for transferring the toner imageformed by the developing device onto a transfer material, and a fixingdevice for fixing the transferred toner image to the transfer material.

According to a third aspect of the present invention, there is providedan image forming apparatus comprising a light scanner according to anyone of the aforementioned claims, and a printer controller forconverting code data input from an external device into an image signal,and inputting the image signal to the light scanner.

According to a fourth aspect of the present invention, there is provideda multi-beam scanner comprising a light source for emitting a pluralityof light beams, a deflector for deflecting the plurality of light beamsemitted from the light source, a scanning optical system for guiding theplurality of light beams that have been deflected by the deflector ontoa scan surface, a first light shielding member for determining one endof a diameter of at least one of the plurality of light beams, and asecond light-shielding member for determining the other end of thediameter of the at least one of the plurality of light beams. The firstlight-shielding member and the second light-shielding member are used tolimit the diameter of the at least one of the plurality of light beamsand are disposed apart from each other in a direction in which the lightbeams propagate.

In one form of the fourth aspect, the first light-shielding member andthe second light-shielding member limit the light-beam diameter within amain scanning cross sectional plane.

In another form of the fourth aspect, the first light-shielding memberand the second light-shielding member limit the light-beam diameterwithin a subscanning cross sectional plane.

In still another form of the fourth aspect, the first light-shieldingmember determines a scan-surface-side portion of the diameter of the atleast one of the plurality of light beams and the second light-shieldingmember determines a portion at a side opposite to the scan-surface sideof the diameter of the at least one of the plurality of light beams. Thesecond light-shielding member is disposed closer to the deflector thanthe first light-shielding member.

In still another form of the fourth aspect, the multi-beam scannerfurther comprises a cylindrical lens disposed between the light sourceand the deflector having a refractive power only in a subscanningdirection. The first light-shielding member and the secondlight-shielding member are disposed between the cylindrical lens and thedeflector.

In still another form of the fourth aspect, of the first and secondlight-shielding members, the light-shielding member that is disposed ata light-source side determines subscanning-direction diameters of theplurality of light beams emitted from the light source.

In still another form of the fourth aspect, the first light-shieldingmember and the second light-shielding member are integrally formed.

In still another form of the fourth aspect, the first light-shieldingmember and the second light-shielding member are disposed in that orderfrom a light-source side. When the distance from a reference position ofa deflecting surface of the deflector to the first light-shieldingmember is L1 (mm), and when the distance from the reference position ofthe deflecting surface of the deflector to the second light-shieldingmember is L2 (mm), the following condition is satisfied:

L2≦0.8×L1.

In still another form of the fourth aspect, the first light-shieldingmember and the second light-shielding member are disposed in that orderfrom a light-source side. When the distance from a reference position ofa deflecting surface of the deflector to the first light-shieldingmember is L1 (mm), and when the distance from the reference position ofthe deflecting surface of the deflector to the second light-shieldingmember is L2 (mm), the following conditions are satisfied:

L2<L1

L2≦20 (mm).

In still another form of the fourth aspect, the multibeam scannerfurther comprises a lens system disposed between the light source andthe deflector. The first light-shielding member and the secondlight-shielding member are disposed between the light source and thelens system. The first light-shielding member and the secondlight-shielding member are disposed in that order from a light-sourceside. The light source includes a plurality of light-emitting points.When the number of the plurality of light-emitting points is n, thepitch in a main scanning direction is d (mm), the focal length of saidlens system is fc (mm), the distance from a reference position of adeflecting surface of the deflector to the first light-shielding memberis L1 (mm), and the distance from the A reference position of thedeflecting surface of the deflector to the second light-shielding memberis L2 (mm), the following condition is satisfied:${\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc}}} \leq {0.2\quad {({mm}).}}$

In still another form of the fourth aspect, the multibeam scannerfurther comprises a lens system disposed between the light source andthe deflector. The first light-shielding member and the secondlight-shielding member are disposed between the light source and thelens system. The first light-shielding member and the secondlight-shielding member are disposed in that order from a light-sourceside. The light source includes a plurality of light-emitting points.When the number of the plurality of light-emitting points is n, thepitch in a main scanning direction is d (mm), the focal length of thelens system is fc (mm), the distance from a reference position of adeflecting surface of the deflector to the first light-shielding memberis L1 (mm), the distance from the reference position of the deflectingsurface of the deflector to the second light-shielding member is L2(mm), and the focal length of the scanning optical system within a mainscanning cross sectional plane is fk (mm), the following condition issatisfied:${\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq {0.01.}$

In still another form of the fourth aspect, the following condition issatisfied:${\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq {0.002.}$

In still another form of the fourth aspect, the multibeam scannerfurther comprises a synchronism detector for detecting synchronism byreceiving the plurality of light beams deflected by the deflector, and asynchronism detecting diaphragm for limiting diameters of the pluralityof light beams incident upon said synchronism detector. The synchronismdetecting diaphragm is disposed between the deflector and thesynchronism detector.

The synchronism detecting diaphragm may limit onlymain-scanning-direction diameters of the plurality of light beamsdeflected by the deflecting means.

The synchronism detecting diaphragm may be integrally formed with eitherone of or both of the first light-shielding member and the secondlight-shielding member.

In still another form of the fourth aspect, the multibeam scannerfurther comprises a synchronism detector for detecting a synchronismsignal of the plurality of light beams used to scan the scan surface asa result of being deflected by the deflector, a third light-shieldingmember for intercepting one end of a diameter of at least one of theplurality of light beams deflected by the deflector, and it fourthlight-shielding member for intercepting the other end of the diameter ofthe at least one of the plurality of light beams deflected by thedeflector. The third light-shielding member and said fourthlight-shielding member are disposed apart from each other in thedirection in which the light beams propagate and between the deflectorand the synchronism detector.

At least one of the first light-shielding member and the secondlight-shielding member and at least one of the third light-shieldingmember and the fourth light-shielding member may be integrally formed.

According to a fifth aspect of the present invention, there is provideda multibeam scanner comprising a light source for emitting a pluralityof light beams, a deflector for deflecting the plurality of light beams,a scanning optical system for guiding the plurality of light beamsdeflected by the deflector onto a scan surface, a synchronism detector,a first light-shielding member for intercepting one end of a diameter ofat least one of the plurality of light beams deflected by the deflector,and a second light-shielding member for intercepting the other end ofthe diameter of the at least one of the plurality of light beams. Thefirst light-shielding member and the second light-shielding member aredisposed apart from each other in a direction in which the light beamspropagate and between the deflector and the synchronism detector.

According to a sixth aspect of the present invention, there is providedan image forming apparatus comprising any one of the multibeam scannersof the fourth aspect and the forms of the fourth aspect, aphotosensitive member disposed at the scan surface, a developing devicefor developing as a toner image an electrostatic latent image formed onthe photosensitive member using the light beams with which thephotosensitive member has been scanned by the multi-beam scanner, atransferring device for transferring the toner image formed by thedeveloping device onto a transfer material, and a fixing device forfixing the transferred toner image to the transfer material.

According to a seventh aspect of the present invention, there isprovided an image forming apparatus comprising any one of the multibeamscanners of the fourth aspect and the forms of the fourth aspect, and aprinter controller for converting code data input from an externaldevice into an image signal and inputting the image signal to themultibeam scanner.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a multibeam scanner of a first embodimentof the present invention in a main scanning direction.

FIG. 2 is a schematic view of the main portion of an aperture diaphragmof the multibeam scanner of the first embodiment of the presentinvention.

FIG. 3 illustrates optical action at the aperture diaphragm of themultibeam scanner of the first embodiment of the present invention.

FIG. 4 is a sectional view of a comparative example of the multibeamscanner of the first embodiment of the present invention in the mainscanning direction.

FIG. 5 is a graph showing the design values of the amount of jitter inthe comparative example and in the first embodiment of the presentinvention.

FIG. 6 is a graph showing the amount of jitter that occurs when a drumis decentered in the comparative example and in the first embodiment ofthe present invention.

FIG. 7 is a graph showing the amount of jitter that occurs when the lastsurface of scanning optical means is decentered in the comparativeexample and in the first embodiment of the present invention.

FIG. 8 is a graph showing the amount of pitch error that occurs when thelast surface of the scanning optical means is decentered in thecomparative example and in the first embodiment of the presentinvention.

FIG. 9 is a sectional view of a multibeam scanner of a second embodimentof the present invention in the main scanning direction.

FIG. 10 is a schematic view of the main portion of an aperture diaphragmof the multibeam scanner of the second embodiment of the presentinvention.

FIG. 11 illustrates optical action at the aperture diaphragm of themultibeam scanner of the second embodiment of the present invention.

FIG. 12 is a sectional view of a multibeam scanner of a third embodimentof the present invention in the main scanning direction.

FIG. 13 is a sectional view of a multibeam scanner or a fourthembodiment of the present invention in the main scanning direction.

FIG. 14 is a schematic view of the main portion of a diaphragm of themultibeam scanner of the fourth embodiment of the present invention.

FIG. 15 is a sectional view of a multibeam scanner of a fifth embodimentof the present invention in the main scanning direction.

FIG. 16 is a schematic view of the main portion of a diaphragm of themultibeam scanner of the fifth embodiment of the present invention.

FIG. 17 is a sectional view in a subscanning direction of an example ofa structure of an image forming apparatus (electrophotographic printer)using any one of the multibeam scanners of the present invention.

FIG. 18 is a schematic view of the main portion of a related multibeamscanner.

FIG. 19 is a sectional view of another related multibeam scanner in amain scanning direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[First Embodiment]

FIG. 1 is a sectional view (main scanning sectional view) of a multibeamscanner of a first embodiment of the present invention in a mainscanning direction.

In the specification, the direction in which deflecting means deflects(reflects) light for performing a scanning operation is defined as themain scanning direction, while a direction which is perpendicular to themain scanning direction and the optical axis of scanning optical meansis defined as the subscanning direction.

Referring to FIG. 1, reference numeral 1 denotes light source means,which is, for example, a semiconductor laser array including twolight-emitting points. There may be three or more light-emitting points.The two light-emitting points are disposed apart from each other in themain scanning direction and the subscanning direction.

Reference numeral 2 denotes a condenser lens system which includes acollimator lens and which converts two divergent light beams that havebeen emitted by the light source means 1 into substantially parallellight beams.

Reference numeral 3 denotes a cylindrical lens which has a predeterminedamount of refractive power only in the subscanning direction, and whichfocuses the two light beams that have passed through the collimator lens2 at a location near a deflecting surface 5 a of a light deflector 5(described later) as a longitudinal linear image in the main scanningdirection.

Reference numeral 4 denotes an aperture diaphragm (diaphragm member)including a first light-shielding member 4 a and a secondlight-shielding member 4 b. The first light-shielding member 4 arestricts one end of the diameter of at least one of the two light beamsthat have passed through the cylindrical lens 3. The secondlight-shielding member 4 b restricts the other end of the diameter ofthe at least one of the two light beams that have passed through thecylindrical lens 3. The first light-shielding member 4 a and the secondlight-shielding member 4 b are disposed apart from each other in adirection of propagation of the light beams between the cylindrical lens3 and the light deflector 5. The diameters of the light beams that areincident upon the deflecting surface 5 a of the light deflector 5 arerestricted using the two light-shielding members 4 a and 4 b.

In the embodiment, the first light-shielding member 4 a determines ascan-surface-side-7 a portion of the diameter of the at least one lightbeam, whereas the second light-shielding member 4 b determines a side-7b portion opposite to the scan-surface-side 7 a of the diameter of theat least one light beam. The second light-shielding member 4 b isdisposed closer to the light deflector 5 than the first light-shieldingmember 4 a. The first light-shielding member 4 a and the secondlight-shielding member 4 b are integrally formed using, for example, asheet metal. However, the first light-shielding member 4 a and thesecond light-shielding member 4 b may be separately formed.

The first and second light-shielding members 4 a and 4 b can restrictthe diameters of the light beams within a main scanning cross-sectionalplane and/or a subscanning cross-sectional plane. In particular, thefirst light-shielding member 4 a restricts the subscanning-directiondiameters of the two light beams emitted from the light source means 1.

The collimator lens 2, the cylindrical lens 3, the aperture diaphragm 4,etc., are each a component part of light-incident optical means.

The light deflector 5, serving as deflecting means, is, for example, arotating polygon mirror having four surfaces, and rotates with aconstant speed in the direction of arrow A (shown in FIG. 1) by drivingmeans (not shown) such as a motor.

The light beams from the light source means 1 may be directly guided tothe light deflector 5 through the aperture diaphragm 4 without using thecollimator lens 2, the cylindrical lens 3, etc.

Reference numeral 6 denotes a scanning lens system (image-formingscanning optical system), serving as scanning optical means, having alight-condensing function and an fθ characteristic. The scanning lenssystem 6 comprises two lenses, first and second optical elements (fθlenses) 6 a and 6 b, having aspherical surfaces. The two light beamsthat have been deflected by the light deflector 5 are focused in theform of spots on a scan surface (photosensitive drum surface) 7 in orderto form two scanning lines. By putting a location near thephotosensitive drum surface 7 and a location near the deflecting surface5 a of the light deflector 5 in a conjugate relationship within thesubscanning cross-sectional plane, the scanning lens system 6 is causedto provide a tilt correcting function.

Reference numeral 7 denotes the photosensitive drum surface, serving asa scan surface.

In the embodiment, the two light beams that have exited from the lightsource means 1 after being modulated in accordance with imageinformation are converted into substantially parallel light beams by thecollimator lens 2, and the substantially parallel light beams areincident upon the cylindrical lens 3. Of the light beam portions whichhave impinged upon the cylindrical lens 3, those within the mainscanning cross-sectional plane exit therefrom unchanged. On the otherhand, those within the subscanning cross-sectional plane are focused toform a substantially linear image (that is, a longitudinal linear imagein the main scanning direction) at the deflecting surface 5 a of thelight deflector 5 through the aperture diaphragm 4. At this time, thesizes of the cross sections of the light beams are restricted by theaperture diaphragm 4. The light beams which have been deflected(reflected) at the deflecting surface 5 a of the light deflector 5 arefocused in the form of spots on the photosensitive drum surface 7 by thescanning lens system 6. By rotating the light deflector 5 in thedirection of arrow A, the photosensitive drum surface 7 is opticallyscanned at a constant speed in the direction of arrow B (that is, themain scanning direction), whereby two scanning lines are formed on thephotosensitive drum surface 7, serving as a recording medium, in orderto perform an image recording operation.

In the embodiment, as mentioned above, the first and secondlight-shielding members 4 a and 4 b are both disposed in an optical pathbetween the cylindrical lens 3 and the polygon mirror 5, so that theinterval between the light beams in the main scanning direction on thedeflecting surface 5 a of the polygon mirror 5 is made small.

FIG. 2 is a schematic view of the main portion of the aperture diaphragm4 used in the first embodiment of the present invention. In FIG. 2,component parts corresponding to those shown in FIG. 1 are given thesame reference numerals.

In FIG. 2, the first light-shielding member 4 a is formed of a flatplate with an elliptical opening 4 a 1 being formed therein, and has ashape which is formed by cutting away a portion thereof disposed at theside 7 b opposite to the scan surface 7 and which restrictsscan-surface-side-7 a ends of the two light beams that have exited fromthe semiconductor laser array 1 (not shown). The second light-shieldingmember 4 b is similarly formed of a flat plate with an ellipticalopening 4 b 1 having the same shape as the elliptical opening 4 a 1being formed therein, and has a shape which is formed by cutting away ascan-surface-side-7 a portion thereof and which restricts ends of thetwo light beams disposed at the side 7 b opposite to the scan surface 7that have exited from the semiconductor laser array 1. Here, the centerof each of the elliptical openings 4 a 1 and 4 b 1 is disposed on theoptical axis of the collimator lens 2. As shown in FIG. 2, the firstlight-shielding member 4 a and the second light-shielding member 4 b areintegrally formed using one sheet plate 4 c.

With reference to FIG. 3, the structure and optical action of theaperture diaphragm 4 used in the embodiment will be described. Partscorresponding to those shown in FIGS. 1 and 2 are given the samereference numerals.

In FIG. 3, the two light beams that have exited from the semiconductorlaser array 1 (not shown) are defined as A and B laser beams. FIG. 3schematically illustrates a state in which, after the A and B laserbeams have been converted into substantially parallel light beams by thecollimator lens 2 (not shown), the substantially parallel light beamsare incident upon the deflecting surface 5 a of the polygon mirror 5through the first and second light-shielding members 4 a and 4 b.

The first light-shielding member 4 a restricts the scan-surface-side-7 aends of both the A and B laser beams, whereas the second light-shieldingmember 4 b restricts the ends of the A and B laser beams disposed at theside 7 b opposite to the scan surface 7. The centers of the light beamdiameters (widths) that have been restricted by the first and secondlight-shielding members 4 a and 4 b are used to define principal rays.The positions of the principal rays are defined as the positions of thelight beams.

Here, the A and B laser beams cross each other at a position Pintermediate between the first light-shielding member 4 a and the secondlight-shielding member 4 b, so that, practically, the aperture diaphragmis disposed at this position. This means that a distance L0 from thedeflecting surface 5 a of the polygon mirror 5 to the practical aperturediaphragm is equal to the average of the sum of a distance L1 from thedeflecting surface 5 a to the first light-shielding member 4 a and adistance L2 from the deflecting surface 5 a to the secondlight-shielding member 4 b as indicated in the following Condition (a):$\begin{matrix}{{L0} = \frac{{L1} + {L2}}{2}} & (a)\end{matrix}$

Since L2<L1 (Condition (b)) and L0<L1 (Condition (c)), it can be seenfrom the numerical conditions that, compared to the position of theaperture diaphragm disposed at the first light-shielding member 4 a in arelated structure, the practical position of the aperture diaphragm issituated closer to the deflecting surface 5 a.

The amount of jitter and the amount of pitch error vary in proportion tothe distance L0 from the deflecting surface 5 a of the polygon mirror 5to the practical position P of the aperture diaphragm 4. Therefore, bymaking use of the advantages of the present invention, a multibeamscanner with reduced jitter and pitch error can be provided.

Accordingly, in the related multibeam scanner, the light beams deflectedat the deflecting surface 5 a of the polygon mirror 5, the scanning lenssystem 6, etc., are disposed close to the deflecting surface 5 a of thepolygon mirror 5, so that, due to physical interference, there is nospace to dispose the first light-shielding member 4 a, disposed at thescan-surface side 7 a, near the deflecting surface 5 a of the polygonmirror 5, thereby making it impossible to dispose the aperture diaphragm4 near the deflecting surface 5 a of the polygon mirror 5. In contrast,if the structure of the embodiment is used, with the light-shieldingmember 4 a, disposed at the scan-surface side 7 a, being disposed at itspresent position, the second light-shielding member 4 b, which isdisposed at the side 7 b opposite to the scan surface and which does notcause physical interference, is disposed close to the deflecting surface5 a of the polygon mirror 5 in order to make it possible to dispose thepractical aperture diaphragm close to the deflecting surface 5 a of thepolygon mirror 5.

Therefore, it becomes possible to properly focus the two light beamsthat have passed through the aperture diaphragm 4 on the scan surface 7,and to decrease jitters and pitch errors.

Here, since angles α and β of the A and B laser beams from an opticalaxis La are different, the diameters of the light beams incident uponthe deflecting surface 5 a are different. However, since the angles αand β are very small, the amount of change in the diameters of the lightbeams is small, so that this is essentially not a problem.

In the embodiment, when the distance from a reference position of thedeflecting surface 5 a of the polygon mirror 5 to the firstlight-shielding member 4 a is L1 (mm) and the distance from thereference position of the (deflecting surface 5 a of the polygon mirror5 to the second light-shielding member 4 b is L2 (mm), the followingCondition (1) is satisfied:

L2≦0.8×L1   (1)

Using the parameters L1 and L2, the following Condition (2) issatisfied: $\begin{matrix}\left. \begin{matrix}{{L2} < {L1}} \\{{L2} \leq {20\quad ({mm})}}\end{matrix} \right\rbrack & (2)\end{matrix}$

In the embodiment, when the number of the plurality of light-emittingpoints is n, the pitch in the main scanning direction is d (mm), and thefocal length of the condenser lens system (collimator lens) 2 is fc(mm), and when the parameters L1 and L2 are used, the followingCondition (3) is satisfied: $\begin{matrix}{{\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc}}} \leq {0.2\quad ({mm})}} & (3)\end{matrix}$

In the embodiment, when the focal length of the scanning lens system 6in the main scanning direction is fk (mm), and when the parameters L1,L2, n, d, and fc are used, the following Condition (4) is satisfied:$\begin{matrix}{{\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq 0.01} & (4)\end{matrix}$

Desirably, the Condition (4) is transformed into Condition (5):$\begin{matrix}{{\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq 0.002} & (5)\end{matrix}$

The technical meaning of the Conditions (1) to (5) will be given.

The Conditions (1) and (2) define the distances L1 and L2. When L1 andL2 do not satisfy at least one of the Conditions (1) and (2), theadvantages of the present invention cannot be satisfactorily exhibited,so that this is not desirable.

The Conditions (3), (4), and (5) define the interval between light beamsat the deflecting surface of the light deflector, considering thestructure of the light-incident optical means extending from the lightsource means to the light deflector. When at least one of the Conditions(3), (4), and (5) is not satisfied, jitter and pitch error become large,so that this is not desirable.

Next, specific numeric examples will be given.

In the multibeam scanner of the embodiment, the number of light-emittingpoints of the semiconductor laser array 1 is n=2, themain-scanning-direction pitch is d=0.09 mm, the focal length of thecondenser lens system (collimator lens) 2 is fc=16.59 mm, the distancefrom the reference position of the deflecting surface 5 a of the polygonmirror 5 to the first light-shielding member 4 a is L1=28.36 mm, thedistance from the reference position of the deflecting surface 5 a ofthe polygon mirror 5 to the second light-shielding member 4 b is L2=7.09mm, and the focal length of the scanning lens system 6 in the mainscanning direction is fk=108.3 mm. In addition, in the multibeam scannerof the embodiment, L2=0.25×L1. Further, the multibeam scanner of theembodiment satisfies the Condition (1) that indicates the range in whichthe advantages of the present invention can be satisfactorily exhibited:

L2≦0.8×L1   (1)

The multibeam scanner of the embodiment also satisfies the Condition 2which indicates the range in which the advantages of the presentinvention can be satisfactorily exhibited: $\begin{matrix}\left. \begin{matrix}{{L2} < {L1}} \\{{L2} \leq {20\quad ({mm})}}\end{matrix} \right\rbrack & (2)\end{matrix}$

In the multibeam scanner of the embodiment,${\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc}}} \leq {0.05\quad ({mm})}$

so that the Condition (3) that indicates the range in which theadvantages of the present invention can be satisfactorily exhibited issatisfied: $\begin{matrix}{{\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc}}} \leq {0.2\quad ({mm})}} & (3)\end{matrix}$

In the multibeam scanner of the embodiment,${\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq 0.0004$

so that the Condition (4) that indicates the range in which theadvantages of the present invention can be satisfactorily exhibited issatisfied: $\begin{matrix}{{\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq 0.01} & (4)\end{matrix}$

In addition, the Condition (5) that indicates the range in which theadvantages of the present invention can be satisfactorily exhibited issatisfied: $\begin{matrix}{{\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq 0.002} & (5)\end{matrix}$

In the embodiment, by satisfying at least one of these Conditions (1) to(5), a multibeam scanner with reduced jitter and pitch error isconstructed.

The structural features of the scanning lens system 6 of the multibeamscanner of the embodiment are given in Table 1.

TABLE 1 STRUCTURE OF MULTIBEAM SCANNER LINE-PERPENDICULAR-LINE-PERPENDICULAR- GENERATING-LINE SHAPE OF fθ LENS GENERATING-LINESHAPE OF fθ LENS TO-GENERATING- TO-GENERATING- 6a 6b LINE SHAPE OF fθLENS 6a LINE SHAPE OF fθ LENS 6b FIRST SECOND FOCAL LENGTH OF SURFACESURFACE IMAGE-FORMING FIRST SECOND FIRST SECOND FIRST SECOND LIGHT LIGHTSCANNING OPTICAL SURFACE SURFACE SURFACE SURFACE SURFACE SURFACE SOURCESOURCE SYSTEM 108.3 LIGHT SOURCE LIGHT SOURCE LIGHT SOURCE LIGHT SOURCELIGHT SOURCE LIGHT SOURCE SIDE SIDE WAVELENGTH IN USE 780 R −2.64814E+01−1.97526E+01 R 8.47991E+01 8.25696E+01 Rs −1.16033E+01 −2.99977E+01 Rs−7.88803E+01 −1.00571E+01 (mm) REFRACTIVE INDEX OF 1.5242 K −1.49902E+00−8.11549E−01 K −8.42997E+00 −8.26049E−01 D2 1.66782E−02 4.74335E−02 D24.13213E−02 1.77203E−03 fθ LENS 6a REFRACTIVE INDEX OF 1.5242 B42.62745E−05 1.30249E−05 B4 −1.54001E−05 −2.19243E−05 D4 −205511B-05−7.89235E−04 D4 −3.82144E−05 −4.56816E−06 fθ LENS 6b B6 −5.63823E−083.59039E−08 B6 1.37412E−08 2.45322E−08 D6 0.00000E+00 5.72932E−06 D6−1.21474E−08 6.29186E−09 POLYGON DEFLECTING 10.50 B8 0.00000E+00−9.03558E−11 B8 −269944E−12 −2.67301E−11 D8 0.00000E+00 −9.37297E−09 D82.14803E−11 −4.13362E−12 SURFACE 5a TO LIGHT-INCIDENT SURFACE OF LENS 6aLIGHT-INCIDENT 6.50 B10 0.00000E+00 0.00000E+00 B10 −2.15513E−152.10166E−14 D10 0.00000E+00 0.00000E+00 D10 0.00000E+00 1.05481E−15SURFACE OF LENS 6a TO LIGHT-EXITING SURFACE OF LENS 6a LIGHT-EXITING7.12 B12 0.00000E+00 0.00000E+00 B12 7.93243E−19 −8.35950E−18 SURFACE OFLENS 6a TO LIGHT-INCIDENT SURFACE OF LENS 6b LIGHT-INCIDENT 6.60 B140.00000E+00 0.00000E+00 B14 0.00000E+00 1.04822E−21 SURFACE OF LENS 6bTO LIGHT-EXITING SURFACE OF LENS 6b LIGHT-EXITING 103.28 B16 0.00000E+000.00000E+00 B16 0.00000E+00 0.00000E+00 SURFACE OF LENS 6b TO SCANSURFACE 7 SIDE AWAY SIDE AWAY SIDE AWAY SIDE AWAY SIDE AWAY SIDE AWAYSIDE AWAY SIDE AWAY FROM FROM LIGHT FROM LIGHT FROM LIGHT FROM LIGHTFROM LIGHT FROM LIGHT FROM LIGHT LIGHT SOURCE SOURCE SOURCE SOURCESOURCE SOURCE SOURCE SOURCE EFFECTIVE SCANNING 214.00 R −2.64814E+01−1.97526E+01 R 8.47991E+01 8.25696E+01 Rs −1.16033E+01 −2.99977E+01 Rs−7.88803E+01 −1.00571E+01 WIDTH ANGLE OF VIEW (deg) 56.24 K −1.49902E+00−8.11549E−01 K −8.42997E+00 −8.26049E−01 D2 −9.72676E−05 −1.03896E−02 D20.00000E+00 1.77203E−03 POLYGON DEFLECTING 134.00 B4 2.62745E−051.22213E−05 B4 −1.71719E−05 −2.31502E−05 D4 −7.39144E−06 8.82172E−05 D40.00000E+00 −4.56816E−06 SURFACE 5a TO SCAN SURFACE 7 POLYGON DEFLECTING30.72 B6 −5.63823E−08 4.20274E−08 B6 1.72463E−08 2.67547E−08 D60.00000E+00 −3.60050E−07 D6 0.00000E+00 6.29186E−09 SURFACE 5a TO LASTSURFACE OF LENS FIRST LIGHT- 28.36 B8 0.00000E+00 −9.98223E−11 B8−4.67025E−12 −2.92126E−11 D8 0.00000E+00 5.30588E−10 D8 0.00000E+00−4.13362E−12 SHIELDING MEANS 4a TO POLYGON DEFLECTING SURFACE 5a SECONDLIGHT- 7.00 B10 0.00000E+00 0.00000E+00 B10 −1.99776E−15 2.29436E−14 D100.00000E+00 0.00000E+00 D10 0.00000E+00 1.05481E−15 SHIELDING MEANS 4bTO POLYGON DEFLECTING SURFACE 5a APERTURE DIAPHRAGM 17.68 B120.00000E+00 0.00000E+00 B12 7.71718E−19 −8.50899E−18 4 TO POLYGONDEFLECTING SURFACE 5a FOCAL LENGTH OF 16.59 B14 0.00000E+00 0.00000E+00B14 0.00000E+00 6.12529E−22 COLLIMATOR LENS POLYGON DEFLECTING ∞ B160.00000E+00 0.00000E+00 B16 0.0000OE+00 0.0000OE+00 SURFACE 5a TOFOCUSING POINT

In the embodiment, the aspherical surface shapes of the first and secondfθ lenses 6 a and 6 b of the scanning lens system 6 within the mainscanning cross sectional plane are represented by Condition (6):$\begin{matrix}{X = {\frac{Y^{2}/R}{1 + \sqrt{1 - {\left( {1 + k} \right)\left( {Y/R} \right)^{2}}}} + {B_{4}Y^{4}} + {B_{6}Y^{6}} + {B_{8}Y^{8}} + {B_{10}Y^{10}} + {B_{12}Y^{12}} + {B_{14}Y^{14}}}} & (6)\end{matrix}$

where the point of intersection of each lens surface and the opticalaxis is defined as the origin, the direction of the optical axis isdefined as the X axis, the axis which is perpendicular to the opticalaxis within the main scanning cross sectional plane is defined as the Yaxis, and the axis which is perpendicular to the optical axis within thesubscanning cross sectional plane is defined as the Z axis. Here, R isthe generating-line curvature radius, and k and B₄ to B₁₄ each representan aspherical coefficient.

The shapes within the subscanning cross sectional plane are representedby Condition (7) used to obtain a radius of curvature r′ in which a lenssurface coordinate in the main scanning direction is Y:

r′=r(1+D ₂ Y ² +D ₄ Y ⁴ +D ₆ Y ⁶ +D ₈ Y ⁸ +D ₁₀ Y ¹⁰)   (7)

Here, r is the radius of curvature of a line perpendicular to thegenerating line on the optical axis, and D₂ to D₁₀ are asphericalcoefficients.

Here, in the case where the coefficients differ due to a difference inthe sign of Y, when Y is positive, the generating-line position X andthe radius of curvature r′ of a line perpendicular to the generatingline are calculated using coefficients to which the letter u is added,B_(4U) to B_(14U) and D_(2U) to D_(10U), respectively. On the otherhand, when Y is negative, the generating-line position X and the radiusof curvature r′ of a line perpendicular to the generating line arecalculated using coefficients to which the letter 1 is added, B₄₁ toB₁₄₁, and D₂₁ to D₁₀₁, respectively.

FIG. 4 is a sectional view (main scanning sectional view) of the mainportion of a comparative example of the embodiment in the main scanningdirection.

In the comparative example shown in FIG. 4, a related multibeam scannerusing an aperture diaphragm 94 formed by one light-shielding member isshown. The comparative example differs from the embodiment in that theaperture diaphragm 94 is disposed at the position of the firstlight-shielding member 4 a used in the embodiment. The other structuralfeatures and optical actions are substantially the same as those of theembodiment.

FIG. 5 is a graph showing the design values of jitter amounts in thecomparative example and in the first embodiment.

As shown in FIG. 5, in the comparative example, the maximum jitteramount is 0.83 μm and the pitch (P—P) is 1.52 μm, whereas, in theembodiment, the maximum jitter amount is 0.51 μm and the pitch (P—P) is0.90 μm. It can be understood that the jitter amount is decreased by theaperture diaphragm 4 that has been disposed by separating the aperturediaphragm 4 into the two light-shielding members 4 a and 4 b in theoptical axis direction and bringing one of the light-shielding members 4a and 4 b close to the polygon mirror 5.

FIG. 6 is a graph showing the jitter amount that is produced when thephotosensitive drum surface 7 (which is a scan surface) shifts towardsthe back by 1 mm along the optical axis of the scanning lens system 6.

As shown in FIG. 6, in the comparative example, the maximum jitteramount is 2.76 μm, whereas, in the embodiment, the maximum jitter amountis 1.72 μm. Accordingly, in the embodiment, the jitter amount that isproduced by decentering of the photosensitive drum is reduced toapproximately two-thirds of the jitter amount in the comparativeexample.

FIG. 7 is a graph showing the jitter amount that is produced when thelast surface of the scanning lens system 6 (the scan-surface-sidesurface of the lens 6 b at the scan-surface side) shifts by 50 μmtowards the scan surface along the optical axis of the scanning lenssystem 6 in the comparative example and in the first embodiment.

As shown in FIG. 7, in the comparative example, the maximum jitteramount is 1.49 μm and the pitch (P—P) is 2.28 μm, whereas, in theembodiment, the maximum jitter amount is 0.93 μm and the pitch (P—P) is1.62 μm. It is possible to decrease the jitter amount that is producedby the decentering of the photosensitive drum and the decentering of thelast surface of the scanning lens system 6 by using the aperturediaphragm 4 used in the embodiment.

FIG. 8 is a graph showing the amount of pitch error that is producedwhen the last surface (the scan-surface-side surface of the lens 6 b atthe scan-surface side) shifts by 50 μm in the subscanning direction inthe first embodiment and in the comparative example.

As shown in FIG. 8, in the comparative example, the maximum pitch erroramount is 1.11 μm and the pitch (P—P) is 2.09 μm, whereas, in theembodiment, the maximum pitch error amount is 0.67 μm and the pitch(P—P) is 1.22 μm. Using the aperture diaphragm 4 used in the embodiment,it is possible to considerably reduce the amount of pitch error that isproduced when the last surface of the scanning lens system 6 isdecentered in the subscanning direction.

In the embodiment, the subscanning-direction diameters of the two lightbeams that have passed through the cylindrical lens 3 are determined bythe first light-shielding member 4 a as mentioned above. In other words,the light beams that have passed through the cylindrical lens 3 areconverged in the subscanning direction, so that, when thesubscanning-direction diameters are determined at a location close tothe light source means 1 where the subscanning-direction diameters ofthe light beams are large, it is possible to reduce the effects causedby the aperture diameter.

Accordingly, in the embodiment, the aperture diaphragm 4 is properlyconstructed and the aforementioned conditions are appropriately set, sothat a multibeam scanner with reduced Jitter and pitch error can beprovided.

[Second Embodiment]

FIG. 9 is a sectional view of a multibeam scanner of a second embodimentof the present invention in the main scanning direction. In FIG. 9,component parts corresponding to those shown in FIG. 1 are given thesame reference numerals.

The second embodiment differs from the first embodiment in the structureof its aperture diaphragm 14. The other structural features and opticalactions are substantially the same as those in the first embodiment,whereby similar advantages are provided.

In FIG. 9, reference numeral 14 denotes the aperture diaphragm includinga first light-shielding member 14 a and a second light-shielding member14 b. The first light-shielding member 14 a determines (restricts) oneend of the diameter of at least one of the two light beams that havepassed through a cylindrical lens 3 (here, the diameter of A laser beamis restricted). The second light-shielding member 14 b restricts theother end of the diameter of the at least one of the two light beamsthat have passed through the cylindrical lens 3. The firstlight-shielding member 14 a and the second light-shielding member 14 bare disposed apart from each other in a direction of propagation of thelight beams between the cylindrical lens 3 and a light deflector 5. Thediameters of the light beams that are incident upon a deflecting surface5 a of the light deflector 5 are restricted using the twolight-shielding members 14 a and 14 b.

In the second embodiment, as shown in FIG. 10, the first light-shieldingmember 14 a is formed of a flat plate including an elliptical opening 14a 1 for restricting the light beams emitted from a semiconductor laserarray (not shown) in the main scanning direction and the subscanningdirection. The second light-shielding member 14 b is formed of a flatplate for intercepting only light beams that are displaced from theoptical axis of a condenser lens system (not shown) towards a side 7 bopposite to a scan surface. The two light-shielding members 14 a and 14b are integrally formed using one sheet plate 14 c. The firstlight-shielding member 14 a and the second light-shielding member 14 bmay be separately provided.

Next, the structure and optical action of the aperture diaphragm 14 willbe described with reference to FIG. 11. In FIG. 11, the two light beamsthat have exited from the semiconductor laser array (not shown) aredefined as A and B laser beams. FIG. 11 schematically illustrates astate in which, after the A and B laser beams have been converted intosubstantially parallel light beams by a collimator lens (not shown), thesubstantially parallel light beams are incident upon the deflectingsurface 5 a of the polygon mirror 5 through the first and secondlight-shielding members 14 a and 14 b. In FIG. 11, component partscorresponding to those shown in FIGS. 9 and 10 are given the samereference numerals.

In FIG. 11, the first light-shielding member 14 a restricts ascan-surface-side-7 a end of the A laser beam, or a scan-surface-side-7a end of the B laser beam and the subscanning direction diameters. Thesecond light-shielding member 14 b restricts only a side-7 b portion ofthe A laser beam opposite to the scan surface.

Here, the A and B laser beams cross each other at a position P disposedtowards the second light-shielding member 14 b from the firstlight-shielding member 14 a by a distance L4 equal to one-fourth of aninterval L3 between the first light-shielding member 14 a and the secondlight-shielding member 14 b, so that the aperture diaphragm ispractically disposed at this position. When the distance from thedeflecting surface 5 a to the first light-shielding member 14 a is L1and the distance from the deflecting surface 5 a to the secondlight-shielding member 14 b is L2, a distance L0 from the deflectingsurface 5 a of the polygon mirror 5 to the practical aperture diaphragmis determined by the following Condition (d): $\begin{matrix}{{L0} = \frac{{3 \times {L1}} + {L2}}{4}} & (d)\end{matrix}$

Since L2<L1 (Condition (e)) and L0<L1 (Condition (f)), it can be seenfrom the numerical conditions that, compared to the position of theaperture diaphragm disposed at a first light-shielding member 14 a in arelated structure, the practical position of the aperture diaphragm isdisposed closer to the deflecting surface 5 a.

As in the first embodiment, the amount of jitter and the amount of pitcherror vary in proportion to the distance from the deflecting surface 5 aof the polygon mirror 5 to the aperture diaphragm 14. Therefore, bymaking use of the advantages of the present invention, a multibeamscanner with reduced jitter and pitch error can be provided.

Compared to the aperture diaphragm 4 used in the first embodiment, theaperture diaphragm 14 used in the second embodiment has a simpler shape,thereby providing the advantages that costs can be reduced and thatdifferences in the diameters of the plurality of light beams in the mainscanning direction can be reduced.

[Third Embodiment]

FIG. 12 is a sectional view of a multibeam scanner of a third embodimentof the present invention in the main scanning direction. In FIG. 12,component parts corresponding to those shown in FIG. 1 are given thesame reference numerals.

The third embodiment differs from the first embodiment in that anaperture diaphragm 24 is formed using one light-shielding member as inthe related examples and that synchronism detecting optical means (BDoptical system) including a BD diaphragm 8 formed by two light-shieldingmembers, a third light-shielding member 8 a and a fourth light-shieldingmember 8 b, is provided. The other structural features and opticalactions are substantially the same as those in the first embodiment,whereby similar advantages are provided.

In FIG. 12, reference numeral 24 denotes the aperture diaphragm formedby one light-shielding member as in the related examples. The aperturediaphragm 24 restricts the diameters of two light beams that have exitedfrom light source means 1.

Reference numeral 8 denotes the synchronism detecting diaphragm(hereinafter referred to as the “BD diaphragm”) and comprises the thirdlight-shielding member 8 a and the fourth light-shielding member 8 b.The third light-shielding member 8 a intercepts one end of the diameterof at least one of the two light beams (BD light beams) that have beendeflected by a polygon mirror 5. The fourth light-shielding member 8 bintercepts the other end of the diameter of the at least one light beam.The third and fourth light-shielding members 8 a and 8 b are disposedapart from each other in the direction in which the light beamspropagate between the polygon mirror 5 and a BD sensor 11 (describedlater).

In the third embodiment, the third light-shielding member 8 a is formedof a flat plate integrally formed with the aperture diaphragm 24. Asdescribed above, the third light-shielding member 8 a restricts one endat a side 7 b opposite to a scan surface of the diameter of the at leastone of the two light beams (BD light beams) deflected by the polygonmirror 5. The fourth light-shielding member 8 b is formed of a flatplate, and restricts the other end of the diameter at a scan-surfaceside 7 a.

Reference numeral 9 denotes a synchronism detecting lens (hereinafterreferred to as the “BD lens”), and gathers light in the main scanningdirection and the subscanning direction.

Reference numeral 10 denotes a synchronism detecting slit (hereinafterreferred to as the “BD slit”), and is disposed at a point where thelight beams (BD beams) are focused by the BD lens 9 or near this pointin order to determine an image write position.

Reference numeral 11 denotes an optical sensor (hereinafter referred toas the “BD sensor”) serving as synchronism detecting means. In theembodiment, using a synchronism signal (BD signal) obtained by detectionof an output signal from the BD sensor 11, a timing at a scanning startlocation for recording an image onto a photosensitive drum surface 7 isadjusted with each BD light beam.

The BD diaphragm 8, the BD lens 9, the BD slit 10, the BD sensor 11,etc., are each component parts of synchronism detecting optical means(BD optical system).

In the embodiment, the two light beams that have exited from asemiconductor laser array 1 after being modulated in accordance withimage information are converted into substantially parallel light beamsby a collimator lens 2, and the substantially parallel light beams areincident upon a cylindrical lens 3. Of the light beam portions whichhave impinged upon the cylindrical lens 3, those within the mainscanning cross-sectional plane exit therefrom unchanged. On the otherhand, those within the subscanning cross-sectional plane are focused toform to form a substantially linear image (that is, a longitudinallinear image in the main scanning direction) at a deflecting surface 5 aof a light deflector 5 through the aperture diaphragm 24. At this time,the sizes of the cross sections of the light beams are restricted by theaperture diaphragm 24. The two light beams which have been deflected(reflected) at the deflecting surface 5 a of the light deflector 5 arefocused in the form of spots on the photosensitive drum surface 7 by ascanning lens system 6. By rotating the light deflector 5 in thedirection of arrow A, the photosensitive drum surface 7 is opticallyscanned at a constant speed in the direction of arrow B (that is, themain scanning direction) in order to perform an image recordingoperation on the photosensitive drum surface 7, which is a recordingmedium.

Here, in order to adjust the timing at a scanning start location on thephotosensitive drum surface 7 prior to scanning the photosensitive drumsurface 7 with light, portions of the two light beams deflected(reflected) at the light detector 5 are focused on the surface of the BDslit 10 by the BD lens 9 through the BD diaphragm 8, after which thefocused portions of the two light beams are guided to the BD sensor 11.Using the synchronism signal (BD signal) that has been obtained by thedetection of the output signal from the BD sensor 11, the timing at ascanning start location for recording an image on the photosensitivedrum surface 7 is adjusted.

In the third embodiment, the semiconductor laser array 1 is rotated withthe optical axis of the collimator lens 2 as a center in order to adjustthe pitch between scanning lines formed by the two light beams to aspecified value. Therefore, the two light beams are separated from eachother in the main scanning direction. When the focal length of the BDlens 9 in the main scanning direction is shifted, the problem that thewrite position is shifted by the BD light beams is created. To overcomethis problem, the BD diaphragm 8 is provided between the polygon mirror5 and the BD lens 9. In a compact multibeam scanner, however, there issometimes no space for disposing the BD diaphragm 8.

In the third embodiment, the problem that there is no space fordisposing the BD diaphragm 8 is overcome by dividing the diaphragm 8into the first and second light-shielding members 8 a and 8 b in orderto allow the BD diaphragm 8 to be disposed at a large number oflocations while retaining the advantages of the BD diaphragm 8.

Although the third embodiment is described by taking the BD diaphragm 8as an example, the aperture diaphragm 24, disposed between the lightsource means 1 and tide light detector 5, may be disposed by separatelyforming two light-shielding members, a first light-shielding member anda second light-shielding member, as in the first embodiment in order toovercome the problem that there is no space for disposing the BDdiaphragm 8. In that case, even if a scanner whose light source meansemits one light beam, or a multibeam scanner whose light source meansemits a plurality of light beams is used, the advantages of the presentinvention can be satisfactorily exhibited as in the third embodiment.

The BD diaphragm 8 may be such as to limit only the light beam diametersin the main scanning direction.

[Fourth Embodiment]

FIG. 13 is a sectional view of a multibeam scanner of a fourthembodiment of the present invention in the main scanning direction. InFIG. 13, component parts corresponding to those shown in FIGS. 1 and 12are given the same reference numerals.

The fourth embodiment differs from the first embodiment in that theincident angle of light from light source means 1 that impinges upon apolygon mirror 5 is changed, and in that a synchronism detecting opticalsystem (BD optical system) including a BD diaphragm 38 is provided. Theother structural features and optical actions are substantially the sameas those in the first embodiment, whereby similar advantages areprovided.

The BD optical system includes the BD diaphragm 38 for restricting atleast one of the two light beams (BD light beams) deflected by adeflecting surface 5 a of a polygon mirror 5, a BD lens 9 for focusingthe light beams near a slit 10, and a BD sensor 11 for detecting light.The BD optical system is used to align write positions on a scan surface7.

In the fourth embodiment, the BD diaphragm 38 is formed of one sheetplate. In addition, first and second light-shielding members 4 a and 4 bof an aperture diaphragm 4 and the BD diaphragm 38 are integrally formedusing one sheet plate as shown in FIG. 14, whereby composite diaphragmmeans is formed.

The first light-shielding member 4 a and the BD diaphragm 38 may beintegrally formed to form the composite diaphragm means.

As in the first embodiment, in FIG. 14, the first light-shielding member4 a is formed of a flat plate with an elliptical opening 4 a 1 beingformed therein, and has a shape which is formed by cutting away aportion thereof disposed at a side 7 b opposite to a scan surface 7 andwhich restricts scan-surface-side-7 a ends of the two light beams thathave exited from a semiconductor laser array 1 (not shown). The secondlight-shielding member 4 b is similarly formed of a flat plate with anelliptical opening 4 b 1 having the same shape as the elliptical opening4 a 1 being formed therein, and has a shape which is formed by cuttingaway a scan-surface-side-7 a portion thereof and which restricts endsdisposed at the side 7 b opposite to the scan surface 7 of the two lightbeams that have exited from the semiconductor laser array 1. Here, thecenter of each of the elliptical openings 4 a 1 and 4 b 1 is disposed onthe optical axis of a collimator lens (not shown).

The BD diaphragm 38 includes an elliptical opening 38 a 1 that isnarrower in the main scanning direction than the elliptical openings 4 a1 and 4 b 1 of the corresponding first and second light-shieldingmembers 4 a and 4 b. The BD diaphragm 38 restricts themain-scanning-direction diameter of at least one of the two light beams(BD light beams) reflected by the polygon mirror 5 (not shown) in orderto make the main-scanning-direction diameters of the two light beamsequal to each other.

When a multibeam scanner in which the aperture diaphragm 4 is formed bythe first and second light-shielding members 4 a and 4 b like thescanner of the fourth embodiment shown in FIG. 13 is used, differencesin the diameters of the two light beams that have exited from the lightsource means 1 cause differences in the intensities of the BD lightbeams incident upon the BD sensor 11, so that an error in detection ofsynchronism (BD detection) may occur.

In such a case, it is possible to adjust the intensities of the lightbeams incident upon the BD sensor 11 so that they are the same bychanging the amount of light emitted from the light source means 1. Asin the fourth embodiment, however, the intensities of the light beamscan be adjusted so that they are the same by providing the BD diaphragm38.

In the structure of the fourth embodiment, by providing the aperturediaphragm 4 formed by the first and second light-shielding means 4 a and4 b, the amount of jitter and pitch error are reduced. By providing theBD diaphragm 38, the problem of the differences in the intensities ofthe two light beams (BD light beams) incident upon the BD sensor 11 isovercome. In other words, when the aperture diaphragm 4, formed by thetwo light-shielding members 4 a and 4 b is used, differences between themain-scanning-direction diameters of the two light beams occur, so thatdifferences between light intensities occur. Therefore, in the fourthembodiment, by causing the main-scanning-direction diameters of thelight beams incident upon the BD sensor 11 to be the same using the BDdiaphragm 38, the light intensities are made constant, so thatsynchronism is stably detected. By this, it is possible to provide amultibeam scanner which always provides a good image.

By integrally forming the first light-shielding member 4 a, the secondlight-shielding member 4 b, and the BD diaphragm 38 using a sheet plateor the like, the number of component parts is decreased, so that reducedcosts and the saving of space are achieved.

[Fifth Embodiment]

FIG. 15 is a sectional view of a multibeam scanner of a fifth embodimentof the present invention in the main scanning direction. In FIG. 15,corresponding component parts to those shown in FIG. 1 are given thesame reference numerals.

The fifth embodiment differs from the fourth embodiment in that a firstlight-shielding member 4 a of an aperture diaphragm 4 is disposed closerto a polygon mirror 5, a BD diaphragm 48 is formed by third and fourthlight-shielding members 48 a and 48 b, and the third light-shieldingmember 48 a is used as the first light-shielding member 4 a. The otherstructural features and optical actions are substantially the same asthose in the fourth embodiment, whereby similar advantages are provided.

In the fifth embodiment, as shown in FIG. 15, with the secondlight-shielding member 4 b being disposed close to a deflecting surface5 a of the polygon mirror 5, the first light-shielding member 4 a isdisposed close to the deflecting surface 5 a. Therefore, the practicalaperture diaphragm position is moved closer to the deflecting surface 5a than in the fourth embodiment by the Condition (a).

The BD diaphragm 48 comprises a third light-shielding member 48 a and afourth light-shielding member 48 b. The third light-shielding member 48a intercepts one end of the diameter of at least one of the two lightbeams (BD light beams) that have been deflected by the polygon mirror 5.The fourth light-shielding member 48 b intercepts the other end of thediameter of the at least one light beam. The third and fourthlight-shielding members 48 a and 48 b are disposed apart from each otherin the direction in which the light beams propagate.

FIG. 16 is a schematic view of the main portion of the aperturediaphragm 4 and the synchronism detecting diaphragm 48 of the multibeamscanner of the fifth Embodiment of the present invention.

In FIG. 16, reference numeral 4 a denotes the first light-shieldingmember and reference numeral 4 b denotes the second light-shieldingmember. The aperture diaphragm 4 is formed by the first and secondlight-shielding members 4 a and 4 b. Reference numeral 48 a denotes thethird light-shielding member, and reference numeral 48 b denotes thefourth light-shielding member. The BD diaphragm 48 is formed by thethird and fourth light-shielding members 48 a and 48 b. Here, the firstlight-shielding member 4 a and the third light-shielding member 48 a areone and the same member, so that space is saved. The BD diaphragm 48only intercepts light in the main scanning direction, and restricts themain-scanning-direction diameter of at least one of the two light beams(BD light beams) deflected by the polygon mirror (not shown), so thatthe shapes of the two light beams that have exited from the BD diaphragm48 are substantially the same and so that their intensities are thesame. This A prevents the occurrence of synchronism detecting errors atthe BD sensor 11 (not shown).

In the structure of the fifth embodiment, while further reducing litterand pitch error by causing the practical position of the aperturediaphragm 4 formed by the first and second light-shielding members 4 aand 4 b to be situated close to the polygon mirror 5, space can be savedby using the first light-shielding member 4 a of the aperture diaphragm4 and the third light-shielding member 48 a of the BD diaphragm 48 asone and the same member. Therefore, it is possible to realize a compactmultibeam scanner with little write position displacements.

Although, in the above-described first to fifth embodiments, the lightbeams that have been emitted by the light source means are convertedinto substantially parallel light beams by the collimator lens, thepresent invention is not limited thereto, so that it is possible tosatisfactorily obtain the advantages of the present invention even ifthe light beams are converted into, for example, convergent light beamsor divergent light beams.

Although, in the above-described first to fifth embodiments, a pluralityof light beams emitted from the semiconductor laser array including aplurality of light-emitting points are converted into substantiallyparallel light beams by one collimator lens, the present invention isnot limited thereto. Accordingly, it is possible to obtain theadvantages of the present invention even by a multibeam scanner whichcomprises a plurality of light source means each including onelight-emitting point and one collimator lens in order to synthesize aplurality of parallel light beams.

Although, in the above-described first to fifth embodiments, a multibeamscanner which emits two light beams is taken as an example, the presentinvention is not limited thereto. The advantages of the embodiments ofthe present invention can be similarly obtained even by a single beamscanner which emits one light beam or a multibeam scanner which usesthree or more light beams.

Although, in each of the above-described embodiments, the scanningoptical means comprises two lenses, the present invention is not limitedthereto, so that the scanning optical means may comprise, for example, asingle lens or three or more lenses.

[Image Forming Apparatus]

FIG. 17 is a sectional view showing the main portion of an embodiment ofan image forming apparatus (electrophotographic printer) in asubscanning cross sectional plane using any one of the multibeamscanners of the first to fifth embodiments. In FIG. 17, referencenumeral 104 denotes an image forming apparatus. Code data Dc is input tothe image forming apparatus 104 from an external apparatus 117, such asa personal computer. The code data Dc is converted into image data (thatis, dot data) Di by a printer controller 111 disposed inside the imageforming apparatus 104. The image data Di is input to a light scanningunit (multibeam scanner) 100 having any one of the structures of thefirst to fifth embodiments. A light beam 103 which has been modulated inaccordance with the image data Di exits from the light scanning unit100. The light beam 103 scans a photosensitive surface of aphotosensitive drum 101 in the main scanning direction.

The photosensitive drum 101 which is an electrostatic latent imagecarrier (a photosensitive member) is rotated clockwise by a motor 115.As the photosensitive drum 101 rotates, the photosensitive surface ofthe photosensitive drum 101 moves with respect to the light beam 103 inthe main scanning direction and the subscanning direction which isperpendicular to the main scanning direction. A charging roller 102which uniformly charges the surface of the photosensitive drum 101 isprovided above the photosensitive drum 101 so that it contactstherewith. The light beam 103 irradiates the surface of thephotosensitive drum 101 charged by the charging roller 102 in order toscan it by the light scanning unit 100.

As discussed above, the light beam 103 is modulated based on the imagedata Di, and, by irradiating the surface of the photosensitive drum 101with the light beam 103, an electrostatic latent image is formedthereon. The electrostatic latent image is developed as a toner image bya developing device 107 which is disposed so as to contact thephotosensitive drum 101 downstream from the location of irradiation bythe light beam 103 within the rotation cross sectional plane of thephotosensitive drum 101.

The toner image which has been formed by developing the electrostaticlatent image by the developing device 107 is transferred onto a sheet112 used as a transfer material by a transfer roller (transfer device)108 disposed below the photosensitive drum 101 so as to oppose it.Although the sheet 112 is contained in a sheet cassette disposed infront of the photosensitive drum 101 (that is, at the right side of thephotosensitive drum 101 in FIG. 17), it may be fed manually. A sheetfeed roller 110 is disposed at an end of the sheet cassette 109, and isused to send the sheet 112 which is contained in the sheet cassette 109into a transportation path.

In this way, the sheet 112 having the toner image transferred but notfixed thereon is transported to a fixing age device which is disposedbehind the photosensitive drum 101 (that is, at the left side of thephotosensitive drum 101 in FIG. 17). The fixing device comprises afixing roller 113 and a presser roller 114. The fixing roller 113 has afixing heater (not shown) disposed therein. The presser roller 114 isdisposed so as to press-contact the fixing roller 113. The sheet 112which has been transported from the transfer section is heated bypressing it at a press-contacting section formed by the fixing roller113 and the presser roller 114 in order to fix the toner image. Asheet-discharge roller 116 is disposed behind the fixing roller 113 inorder to discharge the sheet 112 with the toner image fixed thereon outof the image forming apparatus 104.

Although not illustrated in FIG. 17, the printer controller 111 not onlyconverts the code data Dc but also controls each part, such as the motor115, disposed inside the image-forming apparatus 104, and a polygonmotor and the like disposed inside the light scanning unit 100.

[Diaphragm Member]

The aperture diaphragm including two light-shielding members used in thefirst, second, fourth, and fifth embodiments may be used in variousoptical systems, such as shooting systems, illumination systems, orprojection systems.

According to the present invention, by disposing a portion of thedivided aperture diaphragm near the deflecting means as described above,it is possible to realize a light scanner and a multibeam scanner whichcan reduce jitter and pitch error. It is also possible to realize animage forming apparatus using the same.

In addition, it is possible to achieve a light scanner and a multibeamscanner which allow the aperture diaphragm and the synchronism detectingdiaphragm to be disposed at a larger number of locations. It is alsopossible to achieve an image forming apparatus using the same.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. A light scanner comprising: a deflector fordeflecting light emitted from a light source; a scanning optical systemfor guiding the light deflected by said deflector to a scan surface; afirst light-shielding member for determining one end of a diameterwithin a main scanning cross section plane of the light emitted from thelight source; and a second light-shielding member for determining theother end of the diameter within the main scanning cross section planeof the light emitted from the light source, wherein said firstlight-shielding member and said second light-shielding member are usedto limit the diameter of the light emitted from the light source and aredisposed apart from each other in a direction in which the lightpropagates.
 2. A light scanner according to claim 1, wherein said firstlight-shielding member and said second light-shielding member areintegrally formed.
 3. A light scanner according to claim 1, wherein saidfirst light-shielding member and said second light-shielding memberlimit a diameter of the light within a main scanning cross sectionalplane.
 4. A light scanner according to claim 1, wherein said firstlight-shielding member and said second light-shielding member limit adiameter of the light within a subscanning cross sectional plane.
 5. Animage forming apparatus comprising: a light scanner according to any oneof claims 1 to 4; a photosensitive member disposed at the scan surface;a developing device for developing as a toner image an electrostaticlatent image formed on said photosensitive member using the light withwhich said photosensitive member has been scanned by said light scanner;a transferring device for transferring the toner image formed by saiddeveloping device onto a transfer material; and a fixing device forfixing the transferred toner image to the transfer material.
 6. An imageforming apparatus comprising: a light scanner according to any one ofclaims 1 to 4; and a printer controller for converting code data inputfrom an external device into an image signal, and inputting the imagesignal to said light scanner.
 7. A multi-beam scanner comprising: alight source for emitting a plurality of light beams; a deflector fordeflecting the plurality of light beams emitted from said light source;and a scanning optical system for guiding the plurality of light beamsthat have been deflected by said deflector onto a scan surface; a firstlight-shielding member for determining one end of a diameter within amain scanning cross section plane of at least one of the plurality oflight beams; and a second light-shielding member for determining theother end of the diameter within the main scanning cross section planeof the at least one of the plurality of light beams, wherein said firstlight-shielding member and said second light-shielding member are usedto limit the diameter of the at least one of the plurality of lightbeams and are disposed apart from each other in a direction in which thelight beams propagate.
 8. A multi-beam scanner according to any one ofclaims 7, 10 and 14 to 17, wherein said first light-shielding member andsaid second light-shielding member limit the light-beam diameter withina main scanning cross sectional plane.
 9. A multi-beam scanner accordingto any one of claims 7, 10 and 14 to 17, wherein said firstlight-shielding member and said second light-shielding member limit thelight-beam diameter within a subscanning cross sectional plane.
 10. Amulti-beam scanner comprising: a light source for emitting a pluralityof light beams; a deflector for deflecting the plurality of light beamsemitted from said light source; a scanning optical system for guidingthe plurality of light beams that have been deflected by said deflectoronto a scan surface; a first light-shielding member for determining oneend of a diameter of at least one of the plurality of light beams; and asecond light-shielding member for determining the other end of thediameter of the at least one of the plurality of light beams, whereinsaid first light-shielding member and said second light-shielding memberare used to limit the diameter of the at least one of the plurality oflight beams and are disposed apart from each other in a direction inwhich the light beams propagate, wherein said first light-shieldingmember determines a scan-surface-side portion of the diameter of the atleast one of the plurality of light beams and said secondlight-shielding member determines a portion at a side opposite to thescan-surface side of the diameter of the at least one of the pluralityof light beams, and wherein said second light-shielding member isdisposed closer to said deflector than said first light-shieldingmember.
 11. A multi-beam scanner according to any one of claims 7, 10and 14 to 17, further comprising: a cylindrical lens disposed betweensaid light source and said deflector having a refractive power only in asubscanning direction, wherein said first light-shielding member andsaid second light-shielding member are disposed between said cylindricallens and said deflector.
 12. A multi-beam scanner according to any oneof claims 7, 10 and 14 to 17, wherein of said first and secondlight-shielding members, the light-shielding member that is disposed ata light-source side determines subscanning-direction diameters of theplurality of light beams emitted from said light source.
 13. Amulti-beam scanner according to any one of claims 7, 10 and 14 to 17,wherein said first light-shielding member and said secondlight-shielding member are integrally formed.
 14. A multi-beam scannercomprising: a light source for emitting a plurality of light beams; adeflector for deflecting the plurality of light beams emitted from saidlight source; a scanning optical system for guiding the plurality oflight beams that have been deflected by said deflector onto a scansurface; a first light-shielding member for determining one end of adiameter of at least one of the plurality of light beams; and a secondlight-shielding member for determining the other end of the diameter ofthe at least one of the plurality of light beams, wherein said firstlight-shielding member and said second light-shielding member are usedto limit the diameter of the at least one of the plurality of lightbeams and are disposed apart from each other in a direction in which thelight beams propagate, wherein said first light-shielding member andsaid second light-shielding member are disposed in that order from alight-source side, and wherein when the distance from a referenceposition of a deflecting surface of said deflector to said firstlight-shielding member is L1 (mm), and when the distance from thereference position of the deflecting surface of said deflector to saidsecond light-shielding member is L2 (mm), the following condition issatisfied: L2≦0.8×L1.
 15. A multi-beam scanner comprising: a lightsource for emitting a plurality of light beams; a deflector fordeflecting the plurality of light beams emitted from said light source;a scanning optical system for guiding the plurality of light beams thathave been deflected by said deflector onto a scan surface; a firstlight-shielding member for determining one end of a diameter of at leastone of the plurality of light beams; and a second light-shielding memberfor determining the other end of the diameter of the at least one of theplurality of light beams, wherein said first light-shielding member andsaid second light-shielding member are used to limit the diameter of theat least one of the plurality of light beams and are disposed apart fromeach other in a direction in which the light beams propagate, whereinsaid first light-shielding member and said second light-shielding memberare disposed in that order from a light-source side, and wherein whenthe distance from a reference position of a deflecting surface of saiddeflector to said first light-shielding member is L1 (mm), and when thedistance from the reference position of the deflecting surface of saiddeflector to said second light-shielding member is L2 (mm), thefollowing conditions are satisfied: L2<L1 L2≦20 (mm).
 16. A multi-beamscanner comprising: a light source for emitting a plurality of lightbeams; a deflector for deflecting the plurality of light beams emittedfrom said light source; a scanning optical system for guiding theplurality of light beams that have been deflected by said deflector ontoa scan surface; a first light-shielding member for determining one endof a diameter of at least one of the plurality of light beams; a secondlight-shielding member for determining the other end of the diameter ofthe at least one of the plurality of light beams; and a lens systemdisposed between said light source and said deflector, wherein saidfirst light-shielding member and said second light-shielding member aredisposed between said light source and said lens system, wherein saidfirst light-shielding member and said second light-shielding member areused to limit the diameter of the at least one of the plurality of lightbeams and are disposed apart from each other in a direction in which thelight beams propagate, wherein said first light-shielding member andsaid second light-shielding member are disposed in that order from alight-source side, wherein said light source includes a plurality oflight-emitting points, and wherein when the number of the plurality oflight-emitting points is n, the pitch in a main scanning direction is d(mm), the focal length of said lens system is fc (mm), the distance froma reference position of a deflecting surface of said deflector to saidfirst light-shielding member is L1 (mm), and the distance from thereference position of the deflecting surface of said deflector to saidsecond light-shielding member is L2 (mm), the following condition issatisfied:${\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc}}} \leq {0.2\quad {({mm}).}}$


17. A multi-beam scanner comprising: a light source for emitting aplurality of light beams; a deflector for deflecting the plurality oflight beams emitted from said light source; a scanning optical systemfor guiding the plurality of light beams that have been deflected bysaid deflector onto a scan surface; a first light-shielding member fordetermining one end of a diameter of at least one of the plurality oflight beams; a second light-shielding member for determining the otherend of the diameter of the at least one of the plurality of light beams;and a lens system disposed between said light source and said deflector,wherein said first light-shielding member and said secondlight-shielding member are disposed between said light source and saidlens system, wherein said first light shielding member and said secondlight-shielding member are disposed in that order from a light-sourceside, wherein said first light-shielding member and said secondlight-shielding member are used to limit the diameter of the at leastone of the plurality of light beams and are disposed wart from eachother in a direction in which the light beams propagate, wherein saidlight source includes a plurality of light-emitting points, and whereinwhen the number of the plurality of light-emitting points is n, thepitch in a main scanning direction is d (mm), the focal length of saidlens system is fc (mm), the distance from a reference position of adeflecting surface of said deflector to said first light-shieldingmember is L1 (mm), the distance from the reference position of thedeflecting surface of said deflector to said second light-shieldingmember is L2 (mm), and the focal length of said scanning optical systemwithin a main scanning cross sectional plane is fk (mm), the followingcondition is satisfied:${\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq {0.01.}$


18. A multi-beam scanner according to claim 17, wherein the followingcondition is satisfied:${\frac{d}{2} \times \left( {n - 1} \right) \times \frac{{L1} + {L2}}{2 \times {fc} \times {fk}}} \leq {0.002.}$


19. An image forming apparatus comprising: a multi-beam scanneraccording to any one of claims 7, 10 and 14 to 17; a photosensitivemember disposed at the scan surface; a developing device for developingas a toner image an electrostatic latent image formed on saidphotosensitive member using the light beams with which saidphotosensitive member has been scanned by said multi-beam scanner; atransferring device for transferring the toner image formed by saiddeveloping device onto a transfer material; and a fixing device forfixing the transferred toner image to the transfer material.
 20. Animage forming apparatus comprising: a multi-beam scanner according toany one of claims 7, 10 and 14 to 17; and a printer controller forconverting code data input from an external device into an image signaland inputting the image signal to said multibeam scanner.
 21. Amulti-beam scanner according to any one of claims 7, 10 and 14 to 17,wherein said light source comprises a plurality of light-emitting pointsspaced apart from each other at least in a main scanning direction.